1. Field of the Invention
The present invention relates to a power supply system in which a DC output of a DC power supply whose electric energy increases and decreases, such as solar cells, wind power generators, and fuel cells, is converted to an AC output by a plurality of inverters and is supplied to a system, and concerns a technique for controlling the inverters with high efficiency. In addition, the present invention relates to a parallel-connected system in which electric power generated by a power generating means such as solar cells is converted by inverters to electric power corresponding to a commercial power supply, and is outputted to the commercial power supply.
2. Description of the Related Art
As such a power supply system, a photovoltaic power generation system using solar cells is generally known. FIG. 6 is a system diagram of a conventional photovoltaic power generation system. This photovoltaic power generation system is configured such that a plurality of solar cells (DC power supply) 101 are arranged on the roof of a house, DC outputs generated by these solar cells 101 are collected into one output by a junction box 102, and this DC output is then converted to an AC output through an inverter 103. Subsequently, the power is supplied to the branch circuit inside the house and a commercial-use power system 106 through a distribution board 104. Incidentally, reference numeral 105 denotes an in-house load connected to the branch circuit.
Generally, the inverter has the characteristic that its efficiency declines extremely during a low output. There has been a problem in that if DC/AC conversion is effected by a single inverter in correspondence with the estimated maximum energy generated by the photovoltaic power generation system, the DC/AC conversion efficiency declines during a low output. To solve such a problem, Japanese Patent Application Laid-Open (JP-A) No. 6-165513, for example, discloses a system in which a plurality of inverters with small outputs are connected in parallel, and the number of inverters which are run is increased or decreased in correspondence with the energy generated by the solar cells so as to suppress the decline in the conversion efficiency during a low output.
In addition, in a parallel-connected system, the DC power generated by a generating apparatus such as a photovoltaic power generator is converted to AC power corresponding to a commercial power supply by the inverters, and is then supplied to the commercial power supply.
With the inverters used in such a parallel-connected system, independent operation due to service interruption of the commercial power supply is prevented, and the system interconnection is protected against an overvoltage, an undervoltage, a frequency rise, and a frequency drop in the commercial power supply.
With the inverters used in the parallel-connected system, the most efficient operation is possible during the output of rated power. However, with the power generator using solar cells, since the generated power increases and decreases due to the quantity of solar radiation and the like, the inverters are subjected to maximum power point tracking control (MPPT control) so that the output efficiency becomes highest in correspondence with the increase or decrease in the generated power when the input power is less than the rated power.
As described above, with the inverters whose output power is large, if the input power is excessively low with respect to the rated power, the output efficiency drops extremely. For this reason, a proposal has been made that, with the parallel-connected system, a plurality of inverters be connected in parallel, and the number of driven inverters be set in correspondence with the input power, so that even when the generated power is low, the inverters can be driven efficiently.
With the conventional method, the number of inverters which are driven is determined merely in correspondence with the output power, and no consideration is given to the selection of the inverters which are driven. For this reason, only particular inverters are driven during a low output, and the other inverters are driven only when the output has increased, with the result that the running time of the particular inverters becomes longer than that of the other inverters. Hence, there has been a problem in that the service life of the particular inverters with a long running time expires earlier than the other inverters.
Furthermore, there has been a problem in that if particular inverters among the plurality of inverters are not effective, the overall system fails to work.
In addition, there is a problem in that if the respective output powers of the plurality of inverters are individually controlled, conversely, the conversion efficiency drops depending on the generated power, the number of driven inverters, and so on. Further, if the individual inverters are separately provided with system integration protection when the plurality of inverters are run in parallel, there are cases where their mutual outputs and protective operations interfere with each other, rendering appropriate protection impossible.